Video amplifier circuit for use with synchronous detectors

ABSTRACT

Synchronous video detection in a television receiver provides increased detection linearity and better signal-to-noise ratio for weak signals than does envelope video detection. Impulse noise, which produces generally unobtrusive black-going noise components in a picture reproduced in response to a video signal recovered by envelope detection from a negatively modulated carrier also produces obtrusive white-going noise components in a picture reproduced in a response to a video signal recovered by synchronous detection. Common signal processing means, responsive to the envelope of the video signal modulated intermediate frequency carrier, inverts the white-going noise impulses to produce black-going components so that substantially all noise impulses appear black-going in the recovered signal.

Uite tates George atent [191 1 VIDEO AMPLIFIER CIRCUIT FOR USE WITH SYNCHRONOUS DETECTORS Inventor:

[75] John Barrett George, Indianapolis,

U.S. Cl. 178/7.3 R, l78/DIG. 12, 325/473 Int. Cl. H04n 5/44 Field of Search 178/73 R, 7.5 R, DIG. 12;

[56] References Cited UNITED STATES PATENTS 5/1974 Avins 178/73 R 5/1974 Skerlos l78/DIG. 12

Primary ExaminerRichard Murray Assistant Examiner-Aristotelis M. Psitos Attorney, Agent, or Firm-Eugene M. Whitacre; Mason DeCamillis Jan. 21, 1975 [57] ABSTRACT Synchronous video detection in a television receiver provides increased detection linearity and better signal-to-noise ratio for weak signals than does envelope video detection. Impulse noise, which produces generally unobtrusive black-going noise components in a picture reproduced in response to a video signal recovered by envelope detection from a negatively modulated carrier also produces obtrusive white-going noise components in a picture reproduced in a response to a video signal recovered by synchronous detection. Common signal processing means, responsive to the envelope of the video signal modulated intermediate frequency carrier, inverts the white-going noise impulses to produce black-going components so that substantially all noise impulses appear black-going in the recovered signal.

9 Claims, 6 Drawing Figures SOUND 133 RECOVERY I OIRCUITRY VIDEO PROCESSlNG CIRCUlTRY HORIZONTAL EE R.F. FREQUENCY AMP CONVERTER i lO3 l05 I07 l m l SW P GENERATOR PATENTEU JR"?! I975 SHEET 2 OF 3 PATENTED ms 3; 862.361

' SHEET 30F 3 P ZERO CARRIER WHITE LEVEL T CARRIER VIDEO AMPLIFIER CIRCUIT FOR USE WITH SYNCI-IRONOUS DETECTORS The present invention relates to video amplifier circuits for use in connection with synchronous detection apparatus employed as the video detector of a television receiver and more particularly to apparatus for removing undesired whiter-than-white responses to impulse noise accompanying the television signals, which responses otherwise would appear as imageproducing components in the output signals from the detector.

Synchronous detectors of either the exalted carrier or product detector type are well-known to offer improved detection linearity, freedom from cross modulation and noise immunity as compared to envelope detectors and have consequently been proposed for video detectors in television receivers designed to process amplitude modulated picture carriers. The problem of production in the picture of visible beat frequency components between sound and chrominance subcarriers (e.g. at 920 KHz where NTSC standards are employed) and herringbone beat frequency components between sound and picture information can be reduced by using a synchronous video detector. When a synchronous detector is used, the less stringent intermediate frequency filter requirements for avoiding the sound-chrominance beat permits better phase response in the video i-f amplifier chain. This, in turn, permits better transient response in the chrominance demodulator and amplifier portions of a color television receiver.

In certain types of system employing amplitude modulation of a picture carrier, the behavior of the synchronous video detector during the reception of'impulse noise is less desirable than that of an envelope detector. The envelope type video detector rectifies the peaks of impulse noise such that, if negative picture modulation is employed, black-going spurious signals are produced which generally are considered unobtrusive by the viewer. The synchronous video detector does not rectify impulse noise. Rather, the impulse noise is detected as a spurious midband component, concentrated, for example, around 2 MHZ, which is alternately black-going and white-going. The white-going portions of these spurious noise components are generally considered to be substantially more objectionable to the viewer than the black-going components. Also whiter-than-white signal peaks due to synchronous detection of impulse noise can cause blooming in which the electron beam defocuses, increasing the size of the whiter-than-white spots in the scene and thus making them still more objectionable.

U.S. Pat. No. 2,861,180 describes a principle used to eliminate white-going impulse noise in prior synchronous video detection schemes. Whiter-than-white video signal developed at the output of the synchronous video detector is detected and the video signal is altered to remove the whiter-than-white component. In the cited patent, the video signal is altered by clipping off its whiter-than-white portions.

The methods of detecting whiter-than-white noise and inverting it to black used in British television practice, where positive modulation is employed and envelope video detectors exhibit whiter-than-white impulse noise, also can be adapted for use with synchronous video detectors.

The present invention is embodied in a system including a detector for providing demodulation of carrier waves bearing intelligence encoded in amplitude modulation thereof as provided from a source, which carrier waves are likely to be accompanied by and contaminated with impulse noise. An amplitude modulation detector having an input circuit coupled to said source, has an output circuit for providing recovered intelligence. The detector is responsive to impulse noise whereby the recovered intelligence produced at the output circuit includes impulse noise of first and second polarities relative to said intelligence.

The recovered intelligence which includes impulse noise of first and second polarities is processed in common signal processing means which provides an even number of signal inversions of the recovered intelligence including impulse noise of a first polarity and an odd number of signal inversions to the impulse noise of the second polarity.

The present invention will be better understood by reference to the accompanying drawing in which:

FIG. 1 is a circuit diagram in block form of a television receiver embodying the present invention;

FIG. 2 is a schematic circuit diagram of the common processing means of the present invention;

FIGS. 3a,b,c, are timing diagrams of waveforms'in the television receiver shown in FIG. 1; and

FIG. 4 is a schematic circuit diagram of a video detector system including thepresent invention.

Referring now to FIG. 1, the television receiver has an antenna 101 to intercept broadcast television signals, which are then processed by a radio-frequency (r-f) amplifier 103, a frequency converter 105 typically comprising a mixer and local oscillator combination, and an intermediate frequency (i-f) amplifier 107 to provide composite video signal carrier waves to be detected by a synchronous video detector 109. The composite video-signal detected by the detector 109 is supplied to a syncseparator l 11 via a video amplifier and white spotter 127. The sync separator lll responds to supply separated horizontal and vertical sync pulses respectively to a horizontal sweep generator 113 and a vertical sweep generator 115 to time their sweep waveforms to be properly timed with respect to the received television signals. The sweep waveforms are applied to horizontal and vertical deflection coils 117 associated I with a kinescope 119. The sync separator 111 also supplies separated sync to automatic gain control (AGC) circuitry 120 which controls the gains of the amplifiers 103, 107 to maintain the sensitivity of the detector 109 to received television signals relatively constant. A reference generator 129 is coupled to synchronous detector 109 to provide the demodulation reference, e.g. a 45.75 MHz continuous reference wave for synchronous detection of the modulated incoming video i-f signal from video i-f amplifier 107. A phase detector 110 is coupled between the reference generator 129 and frequency converter 105 to maintain the video i-f amplifier output pass band centered at 45.75 MHZ by control of the frequency converter 105 in a known manner. The output circuit of video amplifier 127 is coupled to the input circuit of video processing circuitry 121 preceding the kinescope 119. The video processing circuitry 121 typically includes luminance channel amplifiers and, in a color receiver, will also include chrominance detector and amplifier circuitry. The illustrated receiver 100 also includes sound recovery circuitry 131 and a speaker 133 to reproduce the accompanying sound information.

A portion of a particular suitable configuration for use in the video amplifier and white spotter 127 is shown in schematic circuit diagram form in FIG. 2. Referring to FIG. 2, an input terminal 212 of video amplifier 127 is coupled to an input transistor 20] having base, collector and emitter electrodes. The collector of input transistor 201 is coupled to a source of voltage +V by a resistor 202. The junction of the collector of transistor 201 and resistor 202 is coupled to the base of a second transistor 204. The emitter of transistor 201 is coupled to the collector of a third transistor 205 which serves as a current source for the input stage 201. The collector of transistor 204 is coupled to the source of operating voltage +V by a resistor 203. The junction of the collector of transistor 204 and resistor 203 is coupled to an output terminal 213 at which an output signal responsive to the input signal is made available. The emitter of transistor .204 is coupled to a point of reference potential (ground) to complete the operational transistor amplifier stage 204. Resistors 206, 207 and 208 associated with transistor 205 provide appropriate biasing to produce the desired current.

In the operation of the apparatus shown in FIGS. 1 and 2, an intermediate frequency carrier wave amplitude modulated with desired video information and undesired noise impulses is supplied to synchronous video detector 109.

FIG. 3a shows a typical horizontal line interval of the modulated video i-f carrier wave as provided by the video i-f amplifier 107 to detector 109. The envelopes of this modulated carrier wave are indicated in outline, the actual variations of carrier wave being omitted since they occur at too high a rate to be clearly represented upon the time scale shown. A noise impulse 50 occurs between times t, and I The time scale in these waveforms has been stretched for the duration of the noise impulse to provide clarity of illustration.

FIG. 3b shows a typical response of the synchronous detector 109 to the modulated video i-f carrier wave shown in FIG. 3a. The noise impulse 50 gives rise to what is shown as a doublet 55 having a black-going portion (which will be referred to as relatively negative) and a white-going portion 57 (which will be referred to as relatively positive). Noise pulses of longer duration will cause multiple ringing in the response of detector 109. The whiter-than-white portion 57 swinging above white level 58 causes obtrusive noise, if not removed, which appears as white-spotting in the reproduced image on kinescope 119.

The signal waveform shown in FIG. 3b is amplified and normally is inverted by transistor amplifier 201.

- Transistor amplifier 204 then provides further amplification and another inversion of the input waveform.

FIG. 3c shows the output of video amplifier and white spotter 127. The black-going portion 56 remains blackgoing, however, the whiter-than-white portion 57 has been inverted only once as noted by the portion 57. To understand how the positive-going pulse is inverted only once, while the remainder of the waveform of FIG. 3b, including the negative-going noise pulse 56, is doubly inverted, it is necessary to examine the input transistor stage 201 under the influence of large negative and positive inputs, representing the noise doublet identified as 56,57 in waveform 3b. When a negativegoing waveform component such as noise pulse 56 is applied to the input transistor 20], it is amplified and inverted in accordance with the gain and limiting characteristics of the transistor stage 201. The amplifier 201 normally is biased for substantially linear amplification of the full range of video signals between sync tips and white level. When a positive-going waveform component such as noise pulse 57 is applied to the input of transistor stage 201, the increasing positive voltage causes transistor 201 to become saturated and transistor 201 becomes forward biased at its basecollector junction. The forward biasing of the basecollectorjunction of transistor 201 produces an equivalent Darlington connection of transistors 201 and 204. That is, the saturated transistor 201 passes the highly positive portion of the input signal without inversion. The total circuit then becomes a singularly inverting stage for the duration of the positive noise pulse which appears in the output waveform as a negative or blackgoing component 57' (see FIG. 30).

Thus, when impulse noise accompanies the carrier waves provided by the intermediate-frequency amplifier 107 to detector 109, the synchronous detector 109 responds with a detected video signal including noise components which swing whiter-than-white. In the absence of any corrective means, the whiter-than-white detector output would result in white spots on the viewing screen of'the kinescope 119.

The video amplifier and white spotter 127 operate as a common signal processor to provide amplification of the video detector output for efficient operation of the video processing circuitry 121 while simultaneously inverting the whiter-than-white noise impulse response of detector output to black, thus preventing the appearance of white spots in the image presented on the face of kinescope 119.

FIG. 4 shows a practical embodiment of the present invention comprising the synchronous video detector 109 coupled to the video amplifier and white spotter 127. Terminals identified as 210, 211, 212, 213 in FIG. 4 correspond to the identically identified terminals in FIGS. 1 and 2. It is convenient to use an integrated circuit 25 for implementation of the desired video amplifier 127. A commercially available integrated circuit suitable for implementing the invention as shown in the schematic circuit of FIG. 2 is the CA 3031/702 integrated circuit amplifier manufactured by RCA Corporation, Somerville, New Jersey. Terminals shown on the amplifier 25 in FIG. 3 correspond to those of the CA 3031/702 device.

To insure proper operation of the amplifier 25 so as to obtain the singularly inverting feature for whitegoing pulses only, attention must be given to biasing, drive levels, and external impedances in utilizing a commercially available integrated circuit such as the CA 3031/702. As shown in FIG. 4 the amplifier 25 is connected and phase compensated in accordance with the manufacturers recommendation.

Lead-lag compensation in the form of a capacitor 27 is used to extend the frequency range of the amplifier to beyond 5 MHz. To achieve a relatively large high frequency output swing capability in the presence of shunt output capacity 28, a resistor 29 is connected from the output terminal 7 to the negative supply point (ground). Input bias and amplifier reference potentials are obtained from a voltage divider including a source (+V) and resistors 31, 32, 33. The amplifier reference potential (terminal 1) is selected typically 0.3 volts more negative than the direct voltage at either of the two signal inputs 2 and 3. The input signal typically may be 100 millivolts peak-to-peak video. Noise pulses of either polarity in the order of several volts amplitude typically may be encountered. The output impedance of detector 109 may be typically 100 to 200 ohms. Such noise pulses are able to drive the amplifier 25 beyond the capability of the included feedback (resistor 24) to cancel the signal current at the inverting input 2. Negative or black-going pulses then reverse bias the input (transistor 201 in FIG. 2) and produce a positive pulse having an amplitude governed by the pulse frequency and amplifier slew rate. Positive or white-going pulses saturate or forward bias the input (transistor 201 in FIG. 2) which produces the desired non-inversion in the input stage. The positive pulse amplitude is similarly governed by pulse frequency and amplifier slew rate.

To summarize the operation of the circuit of FIG. 4, the desired signal output from the detector 109 is inverted and amplified by amplifier 25 (which, in the designated amplifier type, is a differential amplifier including transistors 201, 204 of FIG. 2). Negative-going pulses in the detected signal are inverted and limited by the amplifier while positive-going pulses are not inverted but are limited by the amplifier.

What is claimed is:

1. Apparatus for processing amplitude modulated signals comprising:

a source of carrier waves bearing intelligence encoded in amplitude modulation thereof, said carrier waves likely to be accompanied by and contaminatcd with impulse noise;

an amplitude modulation detector having an input circuit coupled to said source of carrier waves an output circuit for providing recovered intelligence and being responsive to said impulse noise whereby said recovered intelligence produced at said output circuit includes impulse noise of first and second polarities relative to said intelligence;

utilization means for said recovered intelligence; and

common signal processing means coupling said output circuit to said utilization means for providing an even number of signal inversions of said recovered intelligence including said impulse noise of a first polarity and an odd number of signal inversions to said impulse noise of said second polarity.

2. Apparatus for processing amplitude modulated signals as defined in claim 1 wherein said carrier waves are amplitude modulated by a video signal.

3. Apparatus for processing amplitude modulated signals as defined in claim 2 wherein said carrier wave is negatively modulated by said video signal, said video signal having synchronizing signals extending beyond a black level, a white level represented by substantially zero carrier and accompanying noise impulses are black going.

4. Apparatus for processing amplitude modulated signals as defined in claim 1 wherein said amplitude modulation detector is a synchronous detector for recovering said intelligence from said source of carrier waves in the form of amplitude variations-having one polarity with respect to a reference level, said detector producing at its output impulse noise of a first and second polarity with respect to said reference level.

5. Apparatus for processing amplitude modulated signals as defined in claim 1 wherein said common signal processing means includes cascaded amplifier stages wherein a first of said amplifier stages is operated'in a saturated mode for input signals of a second polarity with respect to a reference level and in a substantially linear operating mode for input signals of a first polarity.

6. Apparatus for processing amplitude modulated signals as defined in claim 5 wherein said cascaded amplifier stages include biasing means for operation of said first amplifier in a saturated mode for input signals of said second polarity and substantially linear operation for input signals of said first-polarity.

7. Apparatus for processing amplitude modulated signals as defined in claim 5 wherein said cascaded amplifier stages comprise first and second transistors each having base, emitter, and collector electrodes, the base electrode of said second transistor being coupled to the collector of said first transistor, said first transistor being biased so that signals of a first polarity at the base input of said first transistor drives said first transistor into saturation while signals of a second polarity are processed in a substantially linear operation, said second transistor being coupled as an inverter.

8. Apparatus for processing amplitude modulated signals as defined in claim 4 wherein said synchronous detector output comprises a video signal having synchronizing signals at black level, a white level repre sented by a reference level and the output noise impulse corresponding to the input noise impulse is both black and white going.

9. Apparatus for processing amplitude modulated signals according to claim 5 wherein said common signal processing means comprises a differential amplifier coupled to a signal inverter stage. 

1. Apparatus for processing amplitude modulated signals comprising: a source of carrier waves bearing intelligence encoded in amplitude modulation thereof, said carrier waves likely to be accompanied by and contaminated with impulse noise; an amplitude modulation detector having an input circuit coupled to said source of carrier waves an output circuit for providing recovered intelligence and being responsive to said impulse noise whereby said recovered intelligence produced at said output circuit includes impulse noise of first and second polarities relative to said intelligence; utilization means for said recovered intelligence; and common signal processing means coupling said output circuit to said utilization means for providing an even number of signal inversions of said recovered intelligence including said impulse noise of a first polarity and an odd number of signal inversions to said impulse noise of said second polarity.
 2. Apparatus for processing amplitude modulated signals as defined in claim 1 wherein said carrier waves are amplitude modulated by a video signal.
 3. Apparatus for processing amplitude modulated signals as defined in claim 2 wherein said carrier wave is negatively modulated by said video signal, said video signal having synchronizing signals extending beyond a black level, a white level represented by substantially zero carrier and accompanying noise impulses are black going.
 4. Apparatus for processing amplitude modulated signals as defined in claim 1 wherein said amplitude modulation detector is a synchronous detector for recovering said intelligence from said source of carrier waves in the form of amplitude variations having one polarity with respect to a reference level, said detector producing at its output impulse noise of a first and second polarity with respect to said reference level.
 5. Apparatus for processing amplitude modulated signals as defined in claim 1 wherein said common signal processing means includes cascaded amplifier stages wherein a first of said amplifier stages is operated in a saturated mode for input signals of a second polarity with respect to a reference level and in a substantially linear operating mode for input signals of a first polarity.
 6. Apparatus for processing amplitude modulated signals as defined in claim 5 wherein said cascaded amplifier stages include biasing means for operation of said first amplifier in a saturated mode for input signals of said second polarity and substantially linear operation for input signals of said first polarity.
 7. Apparatus for processing amplitude modulated signals as defined in claim 5 wherein said cascaded amplifier stages comprise first and second transistors each having base, emitter, and collector electrodes, the base electrode of said second transistor being coupled to the collector of said first transistor, said first transistor being biased so that signals of a first polarity at the base input of said first transistor drives said first transistor into saturation while signals of a second polarity are processed in a substantially linear operation, said second transistor being coupled as an inverter.
 8. Apparatus for processing amplitude modulated signals as defined in claim 4 wherein said syncHronous detector output comprises a video signal having synchronizing signals at black level, a white level represented by a reference level and the output noise impulse corresponding to the input noise impulse is both black and white going.
 9. Apparatus for processing amplitude modulated signals according to claim 5 wherein said common signal processing means comprises a differential amplifier coupled to a signal inverter stage. 